The present process relates to an improved additive for introducing magnesium into cast iron melts and to continuous treatment methods for producing ductile cast iron improved by using such additive.
It is well known to produce spheroidal graphite or ductile cast iron by the addition of magnesium as a spheroidizing agent. Since the discovery of this property of magnesium, much effort has been expended in devising safe, inexpensive ways to incorporate and retain the magnesium in cast iron. According to one of the major advances in this art, the magnesium is introduced in the form of an alloy with other metals such as iron, silicon and nickel and combinations thereof. Many nickel-based alloys containing, for example, about 5-15% magnesium have been found useful. The nickel is extremely effective in moderating the reaction between magnesium and molten iron, and it is often a beneficial constituent of the cast iron formed.
Addition alloys are used in many forms depending on the properties of the alloys and the method used to incorporate them into the molten iron. In one method the additives having a density less than that of molten iron are plunged into the melt and react as they rise; in other alloys having greater density than the melt, the additives are dropped into the melt and permitted to sink. The submerged alloys react mainly beneath the surface of the melt and the treatment can be effected in the furnace or the pouring ladle. With the recent emphasis in automation of foundry operations, interest has grown in continuous treatment techniques in making ductile cast iron, for which relatively low reactivity, relatively high density granular additives are particularly suited.
Various techniques for producing ductile cast iron which may be classified as continuous have been proposed. In general the treatment additive is introduced into a stream of molten iron as it flows through a treatment zone. The treatment zone may be a separate vessel or may be a separate area in a given apparatus. In one type of continuous molten iron flows over a bed or pocket or into an enclosed chamber containing the treatment additive and then into a ladle or mold. In another type, a dispensing device injects the treatment additive into a stream of molten iron which subsequently reacts or flows into the ladle or mold. In the "T-NOCK" process, an example of the latter type, the treatment additive is added to the center of a falling stream of molten iron. Continuous treatments are usually performed in a closed chamber, which greatly reduces the inter-action with air but greatly increases refractory erosion - hence the need for a quiet additive. It is also highly desirable for the reaction to be completed in the treatment zone. Particle size is important for achieving optimum performance. Large particles will react too slowly and will tend to clog an injection tube and pouring spout. On the other hand, very fine particles and dust will tend to react violently and to cause a problem termed "blow back" where turbulence induced by the reaction interferes with steady flow of treated iron through the exit spout and may result in rejection of the alloy from the treatment vessel. The very fine powder may also introduce excessive oxygen into the melt and hence reduce magnesium efficiency. This is also undesirable. A useful size for the treatment alloys is roughly rice to pea size, or about 1/8 inch to about 1/4 inch.
It is not new to crush additives to a size suitable for use. For example, a nickel-magnesium-carbon alloy having special utility for the purpose of introducing magnesium into molten cast iron is described in U.S. Pat. No. 2,529,346 and nickel-magnesium-silicon alloyw useful for the same purpose are described in U.S. Pat. Nos. 2,563,859 and 2,690,392. The alloys described in the aforementioned U.S. patents have been prepared by melting and casting the alloys into slabs, crushing the slabs to provide lumps of material which vary considerably in size and shape, and grading the crushed product to provide the lump size ranges desired in iron foundries. The crushing operation employed to produce the alloys in graded particulate form within the desired size range, e.g., 1/8 inch or 1/4 inch or larger lumps, has always resulted in the production of a substantial quantity of fine material. These fines have been found to be of little use for the foundry production of ductile iron since the fines oxidize rapidly in contact with the molten iron with the result that they are ineffective for introducing magnesium in the molten cast iron. Accordingly, these fine materials have been segregated from the desired product and have been remelted to recover the nickel content thereof with accompanying substantial loss of the magnesium content. The presence of fines is particularly objectionable in connection with the continuous treatment processes for the reasons given previously.
A nickel-magnesium-containing alloy has now been found which has properties of crushability, density, reactivity and composition which make it particularly attractive for use in continuous treatment of molten cast iron to produce ductile cast iron. The crushability of the alloys of this invention is such that the desired size can be obtained without generating excessive amounts of fines. Moreover, particles of suitable size can be obtained with conventional crushing equipment, such as jaw crusher, disc pulverizer, roll crusher, etc. Alloys in accordance with this invention have further attributes of low reactivity when added to a cast iron melt, suitably high density, relatively low cost, and they are free of elements which might be detrimental to the production of good ductile iron.
It is an object of the present invention to provide an improved magnesium-containing addition alloy for use in a continuous treatment process for producing ductile cast iron.
It is another object to provide an alloy with controlled crushability characteristics such that particles of the desired size can be obtained without generation of excessive fines.
It is a further object that the alloys provided can be suitably crushed in conventional crushing equipment.
It is still another object that the alloys, in addition to possessing the desired crushability, have low reactivity, high density relative to the melt to which they are added, and low cost, and that they are free of elements which are detrimental to the production of good ductile cast iron.
Other objects and advantages of the invention will become apparent from the accompanying figures and the description which follows.